This invention relates to a process for the preparation of mononitrochlorobenzene by nitration of chlorobenzene using a mixed acid of nitric acid and a concentrated phosphoric acid as the nitrating agent.
Mononitrochlorobenzene having three types of isomers is used as intermediates for dyestuffs, agricultural chemicals and other industrial chemicals.
In conventional methods for the nitration of chlorobenzene, a mixed acid of nitric acid and sulfuric acid has been in general employed as the nitrating agent using the molar ratio of nitric acid to chlorobenzene of about equimol at elevated temperatures. A method using a mixed acid of nitric acid and phosphoric acid is also known.
In the method using the mixed acid of nitric acid and sulfuric acid, due to the dehydrating effect of sulfuric acid on water resulting from the nitration, mononitrochlorobenzene is effectively obtained, however there is a disadvantage that the side reaction to dinitration is promoted with an increase in yields. The formation of dinitrochlorobenzene lowers a purity of mononitrochlobenzene and also, is in danger of exploding during the reaction and subsequent refining steps. Therefore, this by-formation must be controlled to the utmost and the limit of tolerance is 1,000 ppm in general. Accordingly, in case of the nitric/sulfuric mixed acid method a yield relative to nitric acid is controlled to about 95% at most. Also, in this method the nitration reaction product includes a majority of ortho- and para-compounds and a meta-compound of less than 1% and the ratio of para-compound to ortho-compound is about 2.
In the industrial production of mononitrochlorobenzene, it is desirable to alter the isomer ratio in accordance with the demand. It is possible to obtain the para/ortho ratio of less than 2 by selecting reaction conditions. For example, it is known that the para/ortho ratio is reduced with elevating of reaction temperature, however in the nitric/sulfuric mixed acid method the by-formation of dinitrochlorobenzene is still more increased at elevated temperatures and therefore, the nitration at elevated temperatures must be avoided. Thus, in case of the nitric/sulfuric mixed acid method there is a limit in altering the para/ortho ratio.
On the other hand, according to the method using a mixed acid of nitric acid and phosphoric acid as the nitrating agent, the para/ortho ratio is lowered as compared with the case of the nitric/sulfuric mixed acid, but the yield of end products is extremely reduced. If the reaction is carried out under an elevated temperature and pressure to increase the yield, the formation of dinitrochlorobenzene is increased though not so large as in the nitric/sulfuric mixed acid method.
Thus, in a commercial scale, the method using the nitric/sulfuric mixed acid is usually employed.
As to other methods for lowering the para/ortho ratio in the nitration reaction of chlorobenzene, there is provided a method of adding phosphoric acid to the nitrating agent (Japanese Patent Publication No. 52-42783). According to this method, the para/ortho ratio can be successively lowered by increasing the amount of phosphoric acid from 1.63 in case of non-addition to about 1.2. This patent specification discloses a method using a nitric/phosphoric mixed acid in which phosphoric acid is used in the range of 0.90-2.56 molar ratio relative to chlorobenzene and in which, for example, the nitration is carried out using 2 mols of phosphoric acid and 1 mol of nitric acid relative to 1 mol of chlorobenzene, however the yield is only about 80% even at the reaction temperature elevated to 100.degree. C. The reason of this low yield is considered as being as follows:
In the above method orthophosphoric acid (H.sub.3 PO.sub.4) is added in an amount sufficient for using for the mixed acid component as well as altering the para/ortho ratio and therefore, an aqueous phosphoric acid solution of low concentration is present in large quantities in the reaction system. Also, phosphoric acid used instead of sulfuric acid as the mixed acid component is inferior to sulfuric acid in the dehydrating effect on the resulting water increasing with the progress of reaction. Thus the nitration reaction velocity is lowered.
There is provided an improved method of adding such a polyvalent metal catalyst as molybdenum, manganese, vanadium and tungsten to the nitric/phosphoric mixed acid thereby increasing the yield (Japanese Patent Publication No. 52-46928). However, even this method is less than 95% in yield and further, there is an environmental pollution by the waste fluid due to use of the heavy metals.
Besides, there are provided a method using nitric acid and sulfonic acid (U.S. Pat. No. 3,077,502), a method using a solid catalyst having a sulfoxydifluoromethyl group (Japanese Patent Kokai No. 50-154212) and a method using a molecular sieve catalyst and nitrogen dioxide as the nitrating agent in vapour phase (Japanese Patent Kokai No. 54-95521). These methods are disadvantageous in commercial scale in view of costs, complicatedness or yield.